The present invention relates to a moving picture transmission system and a moving picture transmission apparatus used in the system and, more particularly, to a moving picture transmission system for hierarchically coding a moving picture in accordance with the degrees of importance in terms of image quality and transmitting the codes and a moving picture transmission apparatus used in the system.
Hierarchical coding of a moving picture is a coding technique of compressing/coding the moving picture information by dividing the information into blocks in accordance with the degrees of importance in terms of image quality. In the respective moving picture frames constituting moving picture information, energy concentrates at a portion with a low two-dimensional spatial frequency. This portion is significant in terms of image quality.
A scheme of extracting a plurality of pieces of information with different frequency components from each moving picture frame by recursively dividing each moving picture frame in terms of frequency is called sub-band coding.
Japanese Patent Laid-Open No. 8-46960 discloses a coding apparatus for transmitting a multi-resolution moving picture in a multi-speed, multi-format mode by differently coding the respective sub-bands obtained by band division in this scheme, and multiplexing the coding results. Japanese Patent Laid-Open No. 6-339130 discloses a moving picture transmission scheme of ensuring minimum image quality by performing error correction and transmission through two different paths.
According to another typical example of hierarchical coding, in a scheme of dividing a moving picture frame into blocks, performing discrete cosine transform (DCT) in each block, and quantizing the results, each block obtained by DCT is divided into a low-frequency range and a high-frequency range, and the low- and high-frequency ranges of the respective blocks are collected to hierarchically express the moving picture frame. Another method is also available, in which each moving picture frame is hierarchically expressed by using coarsely quantized DCT coefficients and the results obtained by finely quantizing the differences between the original DCT coefficients and the coarsely quantized coefficients.
Japanese Patent Laid-Open No. 2-113748 discloses a scheme of suppressing fluctuations in image quality by dynamically changing the ratio between low-frequency ranges, i.e., most significant parts (MSP), and high-frequency ranges, i.e., least significant parts (LSP) while detecting the congestion of a network in a moving picture transmission system for dividing each DCT block into two blocks, and transmitting them while assigning priorities to the blocks in discarding them in accordance with the congestion of packets in the network.
In addition, Japanese Patent Laid-Open No. 4-100494 discloses a scheme of determining the division of MSP and LSP for each block while estimating influences on image quality, i.e., determining such that pictures to be reproduced from LSP have constant image quality.
These techniques use the following characteristics of moving picture information coded by hierarchical coding. When at least MSP is received at a reception terminal, a moving picture with a certain degree of image quality can be reproduced. When both MSP and LSP are received, a moving picture with high image quality can be reproduced. In addition, the above techniques are used to realize moving picture transmission services with different qualities by using a single bit string or moving picture transmission through a network in which packets may be discarded due to congestion.
The following two problems are posed in the above conventional techniques. In a network such as an Ethernet widely used in offices and the like, when the network is congested, all hierarchical codes cannot be transmitted within a moving picture frame period, and the moving picture frame reception time at a reception terminal becomes longer than the actual frame period. Such prolongation of reception time keeps increasing as long as the congestion of the network continues. As a result, the number of frames to be reproduced per unit time decreases. This prolongs the time required to reproduce moving picture information, affecting the subjective image quality.
Furthermore, in the above schemes, even if the network congestion is settled, the discarded hierarchical codes are not received by the reception terminal. For this reason, no improvement in image quality can be expected even if the network congestion is settled.
To solve these problems, a large memory may be mounted in the reception terminal to allow the reception terminal to start reproduction upon receiving all pieces of moving picture information. In this case, however, long delays occur. This technique cannot therefore be used for real-time communication. In addition, even in a service for which real-time performance is not directly required as in a service of retrieving data from a moving picture database, the user of a reception terminal has to wait for a long period of time, and the reception terminal demands a large memory capacity.